SEISMIC RESPONSE EVALUATION OF THE NPP STRUCTURES AFTER FLOOD EVENT: A CASE STUDY

摘要

This study investigates the effects of deteriorated soil beneath the nuclear island at a Nuclear Power Plant (NPP) following a flooding event. A case study is considered in order to understand potential effects of the negatively affected soil on the structural integrity and seismic response of the combined Class I structures, namely the Auxiliary Building, Containment Shell and Dome Structure, and Containment Internal Structure. The foundation mat for the combined Class I structures is an integral unit supported on steel pipe piles driven to bedrock. The seismic behavior of the nuclear island due to the presence of deteriorated soil under the foundation mat is evaluated by comparing the seismic response that is calculated by using the altered soil properties to the seismic response determined based on the initial design basis criterion for Class I structures. The maximum absolute floor accelerations due to Maximum Hypothetical Earthquake are calculated using response spectrum analysis. The effect of changes in soil properties on the floor response spectra that are used in seismic design criterion for the equipment and piping is also evaluated. In the case study, the original design basis soil-structure dynamic model based on the data provided in the Updated Safety Analysis Report (USAR) is used. The design basis horizontal dynamic model of the NPP structures is reconstituted and validated using a seismic response comparison approach, in which the maximum absolute acceleration response due to Maximum Hypothetical Earthquake and the floor response spectra due to the 1940 El Centra N-S component are compared with the USAR. In the case study, the reconstituted dynamic model is used and modified to predict the dynamic behaviour of the Auxiliary Building and Containment Structure for 10ft depth of the postulated deteriorated soil. The case study demonstrates that even under the extreme postulation that all piles beneath the Auxiliary Building and Containment Structure is surrounded by deteriorated soil and up to the 10ft of pile is completely (laterally) unsupported by the soil, the seismic response of the buildings and structures is less than the original design basis seismic response. The design basis criteria and floor response spectra used for equipment and piping qualification remains unaffected.